FIG. 1 shows an exemplary network structure of an Evolved Universal Mobile Telecommunications System (E-UMTS) as a mobile communication system. The E-UMTS system is a system that has evolved from the existing UMTS system, and its standardization work is currently being performed by the 3GPP standards organization. The E-UMTS system can also be referred to as a Long-Term Evolution (LTE) system.
The E-UMTS network can roughly be divided into an E-UTRAN and a Core Network (CN). The E-UTRAN generally comprises a terminal (i.e., User Equipment (UE)), a base station (i.e., eNode B), an Access Gateway (AG) that is located at an end of the E-UMTS network and connects with one or more external networks. The AG may be divided into a part for processing user traffic and a part for handling control traffic. In this case, the access gateway part that processes the user traffic and the access gateway part that processes the control traffic may communicate with a new interface. One or more cells may exist in a single eNB. An interface may be used for transmitting user traffic or control traffic between eNBs. The CN may include the access gateway and a node or the like for user registration of the UE. An interface for discriminating the E-UTRAN and the CN may be used.
The various layers of the radio interface protocol between the mobile terminal and the network may be divided into a layer 1 (L1), a layer 2 (L2) and a layer 3 (L3), based upon the lower three layers of the Open System Interconnection (OSI) standard model that is well-known in the field of communications systems. Among these layers, Layer 1 (L1), namely, the physical layer, provides an information transfer service by using a physical channel, while a Radio Resource Control (RRC) layer located in the Layer 3 (L3) performs the function of controlling radio resources between the terminal and the network.
To do so, the RRC layer exchanges RRC messages between the terminal and the network. The RRC layer may be located by being distributed in network nodes such as the eNode B, the AG, and the like, or may be located only in the eNode B or the AG.
FIG. 2 shows an exemplary control plane structure of a radio interface protocol between a terminal and an E-UTRAN according to the 3GPP radio access network standard. FIG. 3 shows an exemplary user plane structure of a radio interface protocol between a terminal and an E-UTRAN according to the 3GPP radio access network standard.
The structure of the radio interface protocol between the UE and the E-UTRAN will now be described with reference to FIGS. 2 and 3.
The radio interface protocol has horizontal layers comprising a physical layer, a data link layer, and a network layer, and has vertical planes comprising a user plane (U-plane) for transmitting user data information and a control plane (C-plane) for transmitting control signals. The protocol layers in FIGS. 2 and 3 can be classified into a first layer (L1), a second layer (L2), and a third layer (L3) based on three lower layers of an open system interconnection (OSI) standard model widely known in the communication system. The radio protocol layers exist as pairs between the UE and the E-UTRAN and handle a data transmission in a radio interface.
The layers of the radio protocol control plane of FIG. 2 and those of the radio protocol user plane of FIG. 3 will be described as follows.
The physical layer, the first layer, provides an information transfer service to an upper layer by using a physical channel. The physical layer is connected to an upper layer called a medium access control (MAC) layer via a transport channel. Data is transferred between the MAC layer and the physical layer via the transport channel. The transport channel is divided into a dedicated transport channel and a common channel according to whether or not a channel is shared. Between different physical layers, namely, between a physical layer of a transmitting side and that of a receiving side, data is transmitted via the physical channel.
The second layer includes various layers. First, a medium access control (MAC) layer performs mapping various logical channels to various transport channels and performs logical channel multiplexing by mapping several logical channels to a single transport channel. The MAC layer is connected an upper layer called a radio link control (RLC) layer by a logical channel. The logical channel is divided into a control channel that transmits information of the control plane and a traffic channel that transmits information of the user plane according to a type of transmitted information.
A Radio Link Control (RLC) layer, the second layer, segments and/or concatenates data received from an upper layer to adjust the data size so as for a lower layer to suitably transmit the data to a radio interface. In addition, in order to guarantee various QoSs (Quality of services) required by each radio bearer RB, the RLC layer provides three operational modes: a TM (Transparent Mode); a UM (Unacknowledged Mode); and an AM (Acknowledged Mode). In particular, the RLC layer (referred to as an ‘AM RLC layer’, hereinafter) operating in the AM performs a retransmission function through an automatic repeat and request (ARQ) function for a reliable data transmission.
A Packet Data Convergence Protocol (PDCP) layer of the second layer performs a function called header compression that reduces the size of a header of an IP packet, which is relatively large and includes unnecessary control information, in order to effectively transmit the IP packet such as an IPv4 or IPv6 in a radio interface having a narrow bandwidth. The header compression increases transmission efficiency between radio interfaces by allowing the header part of the data to transmit only the essential information.
The Radio Resource Control (RRC) layer located at the lowermost portion of the third layer is defined only in the control plane, and controls a logical channel, a transport channel and a physical channel in relation to configuration, reconfiguration, and the release of radio bearers (RBs). In this case, the RBs refer to a logical path provided by the first and second layers of the radio protocol for data transmission between the UE and the UTRAN. In general, configuration (or establishment) of the RB refers to the process of stipulating the characteristics of a radio protocol layer and a channel required for providing a particular data service, and setting the respective detailed parameters and operational methods.
If the RRC of the terminal forms a logical connection with the RRC of the wireless network, the terminal is in an “RRC connected mode. Conversely, if there is no logical connection between the RRC of the terminal and the RRC of the wireless network, the terminal is in an “RRC idle mode.”
The Non-Access Stratum (NAS) layer located at the upper portion of the RRC layer performs a function of a session management, a mobility management, and the like.
For the downlink transport channels for transmitting data from the network to the terminal, there are Broadcast Channel (BCH) for transmitting system information, a downlink Shared Channel (SCH) for transmitting user traffic or a control message, and the like. The downlink SCH or a separate downlink Multicast Channel (MCH) may be used to transmit traffic of a downlink MBMS or a control message. Meanwhile, for the uplink transport channels for transmitting data from the terminal to the network, there are Random Access Channel (RACH) for transmitting an initial control message, an uplink Shared Channel (SCH) for transmitting user traffic or a control message, and the like.
The eNB manages radio resources of one or more cells, and one cell is set to one of bandwidths of 1.25, 2.5, 5, 10, 20 MHz so as to provide a downlink or uplink transmission service to a plurality of terminals. Here, different cells may also be configured to provide different bandwidths. The eNB informs the terminals about basic information necessary for an access to the network by using system information (hereinafter, referred to as “SI”). In addition, the eNB may inform the terminals about information of cells (Neighbor Cell List; NCL) adjacent to a cell where the base station provides a service. The system information may include all required information that the terminal should know for a connection with the base station. Accordingly, before the terminal attempts to connect with the base station, it should receive all system information and always have the latest system information. In addition, considering that all terminals within one cell should know the system information, the base station periodically transmits the system information.
Next, description of a cell selection process by a terminal in an idle mode will be given in detail. The cell selection is basically to register for the network such that the terminal receives a service from the base station. Here, if a signal strength or quality between the terminal and the base station becomes deteriorated due to terminal mobility, the terminal would re-select another cell in order to maintain a quality of data transmission. Hereinafter, characteristics of the physical signal, such as the signal strength and a ratio of noise/interference to a signal, may be simply referred to as signal characteristics.
There are methods for selecting or re-selecting a cell according to the signal characteristics depending on the wireless environment. If a cell is to be re-selected, the following cell re-selection methods may be used according to a Radio Access Technology (RAT) of a cell and frequency characteristics.                Intra-frequency cell re-selection: the terminal re-selects a cell having the same RAT and the same center-frequency as a cell currently being used by the terminal.        Inter-frequency cell re-selection: the terminal re-selects a cell having the same RAT and a different center-frequency from the cell currently being used by the terminal.        Inter-RAT cell re-selection: the terminal re-selects a cell using a different RAT from a RAT currently being used by the terminal, or re-selects a cell according to priorities set among different frequencies or RATs.        
FIG. 4 illustrates a procedure of a cell selection operation by a terminal in an idle mode.
S1: The terminal selects a Radio Access Technology (RAT) for a communication with a network (Public Land Mobile Network; PLMN) from which the terminal desires to receive a service. The PLMN and RAT information may be selected by a terminal user or may be stored in the USIM.
S2: The terminal selects a cell having the greatest value among cells whose signal strength with the base station or a quality is determined to be greater than a certain value. Then, the terminal receives SI periodically sent from the base station. The certain value denotes a value defined by the system so as to guarantee a quality for a physical signal during data transmission/reception. Therefore, the certain value may be different according to the RAT to be applied.
S3: The terminal registers its information (e.g., IMSI) to receive a service (e.g., paging) from the network. The terminal does not register for the network desiring to access whenever it selects a cell, but registers for the network if network-related information (e.g., Tracking Area Identity; TAI) received from the SI is different from information which the terminal has.
S4: If a value of a signal strength or quality measured from the base station currently providing a service to the terminal is determined to be smaller than a value measured from a base station of a neighboring cell, the terminal selects one of other cells capable of providing better signal characteristics than the cell of the base station accessed by the terminal. This process is referred to as the cell re-selection, which is distinguished from an initial cell selection. Here, in order to prevent frequent occurrence of cell re-selections due to the changes of the signal characteristics, there is a time restriction. In the LTE system, targets of the signal measurement may include a Reference Symbol Received Power (RSRP), Reference Symbol Received Quality (RSRQ), and Received Signal Strength Indicator (RSSI).
Hereinafter, descriptions of a cell selection method and procedure thereof in WCDMA will be given in detail.
conditions of the cell selection:[Formula 1]    Srxlev > 0 AND Squal > 0Where:  Squal = Qqualmeas − Qqualmin  Srxlev = Qrxlevmeas − Qrxlevmin − PcompensationParameterDescriptionSqualCell Selection quality value (dB)SrxlevCell Selection RX level value (dB)QqualmeasMeasured cell quality value expressed inCPICH Ec/N0 (dB)QrxlevmeasMeasured cell CPICH RSCP RX level value (dBm).QqualminMinimum required quality level in the cell (dB)QrxlevminMinimum required RX level in the cell (dBm)PcompensationRF dependent value (dB)
[Formula 1] indicates conditions of the cell selection by the terminal in WCDMA.
When the terminal is initially turned on, the terminal selects PLMN and RAT for a wireless communication. In the initial cell-selection corresponding to S2 in FIG. 4, the terminal selects and accesses a cell having the greatest signal characteristic value, among cells which meet the conditions of [Formula 1] through the signal measurements with the base station in all searchable frequency bandwidths. In the WCDMA system, target values of the signal measurement may include CPICH RSCP, CPICH Ec/NO and Carrier RSSI.
As shown in [Formula 1], the terminal selects a cell whose measured signal strength and quality are greater than a specific value (strength: Qrxlevmin+Pcompensation, quality: Qqualmin) defined by the system. Here, the Qrxlevmin, Qqualmin, Pcompensation values are values notified by the base station to the terminal through SI. Then, the terminal waits in the idle mode so as to request a service from the network (e.g., originating a call) or to receive a service from the network (e.g., terminating a call). The terminal in the idle mode repeats a process of re-selecting the cell which has better signal characteristic through signal measurements of a serving cell and a neighboring cell. In this instance, if a signal characteristic value of a serving cell is greater than a specific value (e.g., Ssearch), the terminal does not need to perform the cell re-selection, thereby not performing the measurement.
FIG. 5 is a flowchart showing a cell re-selection method in WCDMA with respect to S4 in FIG. 4. Referring to FIG. 5, through periodic measurements, the idle terminal selects a cell corresponding to Rn if a cell having the greatest characteristic value meets a condition of Rn>Rs for a specific period of time (Treselection*) through a ranking process for comparing a signal strength and quality, among cells whose signal characteristic values (Rs) of serving cells and signal characteristic values (Rn) of neighboring cells meet the conditions of [Formula 1]. That is, the terminal selects another cell (Rn) having the better signal characteristic than the serving cell (Rs). Rs and Rn are the values obtained through the calculation process of [Formula 1].Rs=Qmeas,s+Qhysts+QoffmbmsRn=Qmeas,n+Qoffsets,n+Qoffmbms  [Formula 2]
[Formula 2] is used by the terminal for the ranking process among cells in WCDMA.
In [Formula 2], Qmeas,s denotes a CPICH Ec/NO value of a serving cell measured by the terminal, and Qmeas,n denotes CPICH Ec/NO value of neighboring cells measured by the terminal. Qhysts is used by the terminal to apply weight to a serving cell. Qoffsets,n may be used to have a bias between a cell currently connected and a cell to be moved, and Qoffmbms may be used to apply weight to a cell which supports a Multimedia Broadcast Multicast Service (MBMS) service.
Meanwhile, the value “Treselection*” is used to put a restriction that the conditions of the cell re-selection should be met more than a certain period of time so as to prevent the terminal from repeatedly selecting a specific cell. The value “Treselection*” is determined by a Treselection' value transmitted from the base station to the terminal through SI and a speed determined by the terminal. Description of the operation of the terminal to determine such Treselection* will be given in detail with reference to FIG. 6.
FIG. 6 is a flowchart illustrating a method for determining a speed by a terminal after cell re-selection in WCDMA.
In the present invention, Treselection' represents a time restriction received from the base station, and ‘Treselection* represents a value obtained by applying a scaling factor to the Treselection’ of the terminal.
In FIG. 6, if a frequency of a cell selection for a specific period of time (TCRmax) is greater than a certain value (NCR), it considers itself as a high mobility UE. Conversely, if a frequency of a cell selection does not meet the condition of the NCR, it considers itself as a low mobility UE. If the condition for the high mobility cannot be satisfied for a certain period of time (TCRmaxHyst) even after it has been determined as the high mobility UE, it is again determined as the low mobility UE. If high mobility is finally determined, the terminal multiplies the Treselection value by a scaling factor (a value having units of 0.1 between 0 and 1) and thereby to determine a selection time according to a speed at the cell re-selection. That is, if the frequency of the cell change is high, the terminal determines that the speed gets faster, and reduces the time restriction in the cell re-selection so as to make the cell re-selection faster, thereby receiving a service in a cell having good signal characteristics, thus to minimize an error in interpretation of a signal transmitted by a transmitter. The values of TCRmax, NCR, TCRmaxHyst, Treselection and Scaling Factor are provided by the base station through SI, and the table below shows values selected by the base station for transmission.
ParameterValue typeTCRmaxEnumerated (not used, 30, 60, 120,180, 240 sec) (sec: second)NCRInteger (1 . . . 16)TCRmaxHystEnumerated (not used, 10, 20, 30,40, 50, 60, 70 sec)TreselectionInteger (0~31), secScaling Factor for TreselectionReal (0~by step of 0.1)
In addition, the LTE system has specific reference values (NCR_M, NCR_H) for detecting one or more speeds, thus to facilitate speed detection of 3 stages.